The time is right for spider mites

Hot summer days are tough on plants. It’s even worse when things are dry. This is when spider mites attack. We often think of spider mites on herbs, perennials, and vegetables in summer landscapes and especially in greenhouse and nursery production. Twospotted spider mites are most common in these settings and can be very damaging.

What many people don’t think about is spider mites on trees. In fact, most tree species are fed upon by spider mites and many even have their own species. These include maple spider mites, magnolia spider mites, honeylocust spider mite, and many others. This week I have noticed damage on many pin oaks (Quercus falcata) by, you guessed it, oak spider mites (Oligonychus bicolor).

Oak spider mites look much like other mites in the genus Oligonychus, which includes the wide-spread pests southern red mite (O. ilicis) and spruce spider mite (O. ununguis). They are dark red to brown with oval to round bodies and orange legs.

Oak spider mites overwinter as eggs in the nooks and crannies of bark on their host trees. They have many generations per year and can feed on oaks and related trees such as chestnut, hickory, maple, beech, and elm. Interestingly, to me at least, most of the mites in this genus, oak spider mite included, feed on the upper surface of leaves. Many other mites and herbivores feed on the lower surfaces where they seem more protected from UV, rain, dust, and maybe predators.

Oak spider mite damage on oak leaves. Photo: Steve Frank

Pin oaks seem especially susceptible to oak spider mites, as that is where I have seen the most damage lately, but willow oaks also can be heavily damaged. We found that damage increases with the temperature and amount of impervious surface cover around the tree.

Trees that are heavily infested in summer will have many eggs in the winter which will hatch to infest the same tree next year. Thus, the same trees end up damaged repeatedly. Overwintering eggs may be susceptible to horticultural oil applications. Water trees to reduce stress and don’t overdo it on the fertilizer. It just makes trees more nutritious.